Currently, mobile communication system provides service by segmenting an entire service area into a plurality of radio zones each referred to as a “cell”. In a case where a mobile station moves across different cells in the mobile communication system, a “cell change” is performed. In a “cell change,” the mobile station changes a communicating base station with which the mobile station performs radio communication (e.g., see Non-patent literature 1).
At the time of the cell change, a control station controlling the base station switches the paths for downlink data, where the destination of downlink data transmission is switched from a source base station to a target base station. In addition, the target base station transmits, to the control station, a traffic assignment signal, which is a notification of a processable traffic amount of the target base station. According to the notified traffic amount, the control station transmits downlink data to the target base station.
However, the switching of paths requires a long period of time in cases, such as when processing load in the control station is high. During the switching of the paths, the control station is unable to process a traffic assignment signal transmitted from the target base station. For this reason, in some cases the control station fails to transmit, to the target base station, downlink data according to the processable traffic amount of the base station. Thus, a “non-communication” state occurs, where no communication is made.
Conventionally, two methods have been employed in order to release this non-communication state.
In one method, the base station retransmits the traffic assignment signal in a case where downlink data is not transmitted from the control station after elapse of a predetermined period of time.
In the other method, the control station transmits, to the base station, a request signal requesting for the traffic assignment signal, in a case where the control station has not received the traffic assignment signal from the base station after elapse of a predetermined period of time. Thereafter, the base station retransmits the traffic assignment signal in response to the request signal.
However, although being capable of releasing the non-communication state, the conventional methods are incapable of preventing the non-communication state from lasting for a certain period of time.
Hence, in order to allow a control station to smoothly process the traffic assignment signal and to accordingly reduce the non-communication period even when the switching of paths requires a long period of time, conceivable methods are: one in which a base station constantly transmits the traffic assignment signal at a high frequency; and another in which a control station constantly transmits request signals requesting for a traffic assignment signal at a high frequency.
However, the use of these methods have caused new problems, such as increase of the load for signal processing in the base station and in the control station, and loss of communication resource between the base station and the control station. Non-patent Literature 1: 3GPP TS25.214 v5.10.0 (2004-12)